The present invention is directed to protective coatings which may be provided on a wide variety of substrate types, and which are both stable at high temperatures and, in the case of metal substrates, corrosion-resistant. Although a number of materials and procedures for making coatings are known in the art, there remains a need for coatings which are chemically and physically stable to heat and do not corrode over time. Further, many prior art techniques involve very long curing times and/or very high curing temperatures, limiting ease of manufacturability. Frequently, there are trade-offs between the various desired characteristics of flexibility, hardness, and coating adhesion. In addition, most coatings which have one or more of the aforementioned advantages do not provide "release surfaces," i.e. they are not fluid-resistant.
Some organometallic and inorganic polymers are known for their thermal stability. Certain polysiloxanes, for example, have been the most widely commercialized of these polymers, but there has been nothing to suggest that such polymers would be useful to make high temperature coatings, with the exception of a few polymers that were found to be costly to manufacture and were thus never commercialized. Since polysiloxanes have typically been used as high temperature oils and elastomers, very little research has been conducted to modify these compounds to provide hard, relatively thick coatings.
Polyphosphazenes belong to another category of inorganic polymers with potential stability at temperatures of 400.degree. C. or higher. Again, there has not been any suggestion that such polymers could be used as high temperature protective coatings; most research efforts have concentrated on the elastomeric, electrical or optical properties of the polymers.
Still another family of polymers which have been used to provide oxidation-resistant and corrosion-resistant coatings are fluorocarbons such as tetrafluoroethylene, commercially available as Teflon.RTM.. Information concerning tetrafluoroethylene polymers may be found, inter alia, in U.S. Pat. No. 2,230,654 to Plunkett, issued 4 Feb. 1941. Tetrafluoroethylene coatings, however, like coatings prepared from other poly(fluorocarbons), have limited stability at temperatures above about 300.degree. C.
A different approach for protecting metal surfaces against corrosion at high temperatures and in harsh environments is by the application of ceramic coatings. Ceramic coatings can be fabricated at the surface of metals as uniform, hermetically sealed layers that are well-bonded to the substrate. This approach provides another method of protecting metals against chemical attack. However, only thin films can be formed by single-layer deposition without cracking. Additionally, the equipment which has typically been necessary to prepare ceramic coatings is costly, can only process substrates of a limited size, and requires deposition times which are often long.
Preceramic polymers that can be fabricated like organic polymers and then cured and pyrolyzed to give ceramic products are being developed as an alternative for processing advanced ceramics. Thin ceramic coatings can be made by simple wet techniques using solutions of organometallic precursors. The developed coatings are hard, very stable at high temperatures, and provide protection against corrosion. However, these coatings lack flexibility because their extensive crosslinking network results in a high modulus of elasticity. Thick layers (on the order of 4 .mu.m or greater, more typically in the range of about 10 .mu.m to 50 .mu.m) cannot be obtained by a single deposition operation using preceramic polymers because the coatings tend to crack as a result of the drastic shrinkage that occurs during conversion of the polymer to a ceramic network and, further, because of a mismatch in the expansion coefficient between the coating and substrate. It is possible to limit shrinkage to one dimension, vertical to the surface, only when fabricating relatively thin layers (less than 4 .mu.m).
It has now been discovered that selected silicon-containing polymers can be used to prepare coatings which overcome the disadvantages of the prior art and meet all of the above-mentioned criteria, i.e., the coatings provided by the method described and claimed herein are heat-stable, rapidly cured at relatively low temperatures, display excellent hardness and adhesion, can remain intact even when the substrate is deformed, and do not corrode over time. Additionally, the coatings may be prepared under conditions which render them nonwetting.
The following references relate generally to polymeric coatings which are thermally stable and/or nonwetting, and to polymers which can be used to prepare such coatings.
U.S. Pat. No. 3,944,587 to Katsushima et al. describes certain hydroxypolyfluoroalkyl-containing silane derivatives as water- and oil-repellent agents. The reference states that a variety of material types may be rendered water- and oil-repellent by applying coatings of the disclosed silane derivatives. The silane compounds react with the substrate surface to provide the water- and oil-repellent coatings.
U.S. Pat. No. 3,979,546 to Lewis describes a method for rendering inorganic substrates hydrophobic which involves treating the substrate surface with alkoxy-omega-siloxanols. The siloxanols are prepared by reacting selected cyclic siloxanes with alcohols.
U.S. Pat. No. 4,301,197 to Franz et al. describes the use of selected poly(alkyl hydrogen siloxanes) to treat glass surfaces to improve the release of polymeric materials.
U.S. Pat. No. 4,591,652 to DePasquale et al. describes certain polyhydroxyl silanes or siloxanes as useful in preparing coatings on metal or glass. The coatings are prepared by curing at temperatures in the range of 90.degree. C. to 150.degree. C.
U.S. Pat. No. 4,954,539 to Cavezzan et al. describes thin films of an aqueous silicone emulsion crosslinked by a monochelate of pentacoordinated tin and cured at temperatures in the range of 80.degree. to 220.degree. C. The films are stated to be water-repellent and/or nonadhesive.
U.S. Pat. Nos. 4,983,459 and 4,997,684 to Franz et al. describes treatment of a glass surface with a combination of a perfluoroalkyl alkyl silane and a fluorinated olefin telomer to provide a nonreactive, nonwetting surface.
P. Hergenrother, Angew. Chem. Int. Ed. Engl. 29:1262-1268 (1990), generally relates to thermally stable polymers--including polyimides, poly(aryl ethers) and imide/aryl ether copolymers--and their potential uses.
Silicon-containing polymers are described as potentially useful materials in environments which require thermal stability and oxidation-resistance by R. E. Burks, Jr., et al., J. Poly. Sci. 11:319-326 (1973), C. U. Pittman, Jr., et al., J. Poly. Sci. 14:1715-1734 (1976), and P. Dvornic et al., polymer 24:763-767 (1983).